Thermal surfacing i.e. thermal spray coating and overlay welding powder grades are widely used for coating of component surfaces against wear and corrosion. Fe-, Ni- and Co-based grades are known to radically improve life time of wear- and/or corrosion exposed components. However, there is still a large number of applications where component life times need to be improved. In addition, high prices and limited availability of Ni and Co on the world market also calls for longer life time improvement. Finally, development of new coating deposition methods like laser cladding, cold spraying and high velocity spraying open new possibilities for alloying, more accurate control of coating process and higher automation, thereby calling for additional types of powders.
A potential approach to improve friction and wear properties may be to incorporate solid lubricant to thermal surfacing grades so that the deposited coating includes friction and wear reducing substances while maintaining acceptable levels of corrosion resistance and hardness.
Solid lubricants are soft solid phase materials which are capable of reducing friction and wear between two surfaces sliding against each other without the need for a liquid media. Materials to be considered as solid lubricants need to meet at least the following criteria: adhere contacting surfaces—stickiness: low shear strength—low intrinsic friction; low hardness—low abrasivity and thermochemical stability for the intended environment. Examples of solid lubricants are; talc, graphite, manganese sulphide (MnS), molybdenum disulphide (MoS2), or tungsten disulphide (WS2). Use of solid lubricants may provide advantages in: stability at extremely low or high temperatures; stability in extreme environments, such as cold or hot environments, or environments having high radiation levels; mechanical design issues (lighter design, reduced critical velocity) or able to carry extreme loads.
For a long time, the use of solid lubricants in thermal surfacing has been a difficult proposition, the reason being that numerous solid lubricants are metal sulphides and that even trace amounts of sulphur in welds can lead to cracking and/or corrosion.
Skarvelis et al; ASME J. Tribol. 132 (2010) 031302-1-031302-8, Surf. & Coat. Techn. 203 (2009) 1384-1394, and Trib. Int. 42 (2009) 1765-1770 describe the use of mixing MnS powder with a metal powder and using the resulting powder mix in e.g. PTA (plasma transferred arc welding).
Solid lubricants, however may have high friction coefficient compared to that of oil or grease; finite wear life for solid lubricant films when renewal is not possible; no or limited cooling capacity compared to oil or grease, or tendency to clogging caused by debris and residual particles.